In a sequence of a BICOM (or complementary bipolar) fabricating process with separated emitters, a PNP-type semiconductor structure is manufactured while an NPN-type semiconductor structure is already present on the wafer. The NPN-type structure, especially the NPN-emitter provides a surface topography having flat surfaces as well as vertical surfaces. During production of a p-emitter of the PNP-type semiconductor structure, a P-doped poly silicon layer is deposited on top of the aforementioned topography. This p-emitter layer comprises an oxide layer typically of SiO2 that has to be removed during further steps of the semiconductor production process.
FIG. 1 illustrates a typical situation during a BICOM process, wherein p-emitter poly silicon (Pemit Poly) is deposited on top of a first oxide layer (Oxide 1) of an NPN-type emitter (NPN-Emitter) of the earlier deposited NPN-type semiconductor structure. The P-emitter structure itself comprises a second surface oxide layer (Oxide 2) having a bottom antireflective coating (BARC) on top. According to the prior art, in a P-emitter poly etching step, the P-emitter poly layer (Pemit poly) and the second oxide layer (Oxide 2) are removed in a P-emitter poly etching step and parts of the wafer that are not covered by a respective resist are laid open. Especially, the already manufactured NPN-emitter has to lay open after this etching process. It is a task of the respective P-emitter poly etching process that the poly silicon layer (Pemit Poly) and the second oxide layer (Oxide 2) are removed residue-free. According to the prior art, after a BARC-etching step, the P-emitter poly layer (Pemit Poly) is removed in a wet chemical process that is however not preferable for small structures since the resist may lift-off and semiconductor structures that should be protected by the respective resist may be destroyed.
A second process that is known in the prior art is to remove the second oxide layer (Oxide 2) by an oxide plasma etching process. A typical resulting semiconductor structure is shown in FIG. 2. However, since the oxide plasma etching process which is a type of ion etching process has a strong anisotropic behavior, lateral surfaces are removed very quickly while vertical surfaces remain. Accordingly, there is an oxide residue 4, which is a part of the second oxide layer (Oxide 2) that is not removed during this process.
This oxide residue 4 causes that during removal of the P-emitter poly silicon (Pemit Poly) a poly stringer 6 remains (see FIG. 3). These poly stringers 6 cause leakage currents or may even lift off and float over the wafer and are therefore undesired.